1. Field of the Invention
The present invention relates to a mobile communication system, a radio base station apparatus, and a method for determining uplink reception synchronization. More specifically, the invention relates to a method for determining synchronization in uplink reception compressed mode.
2. Description of the Related Art
Radio base station apparatus in accordance with IMT-2000 (International Mobile Telecommunications-2000) standards adopt compressed mode. Compressed mode is set by a higher-layer apparatus to take measurements in handover between different frequencies with other systems, for example, FDD (Frequency Division Duplex), TDD (Time Division Duplex), and GSM (Global System for Mobile communications) (for example, refer to 3GPP TS25.133, 8, “UE Measurements Procedures”).
Compressed mode is a function with which a cell of a different frequency can be measured to perform handover between different frequencies. Single-receiver mobile stations should essentially support downlink compressed mode.
Transition to compressed mode is determined by UTRAN (Universal Terrestrial Radio Access Network), which then informs a mobile station of parameters required in compressed mode.
In compressed mode, data is not transmitted during slots called a transmission gap. To prevent quality degradation due to reduced gain during this halt of transmission in terms of BER (Bit Error Rate) and BLER (BLock Error Rate), transmission power is temporarily increased for frames that are in compressed mode.
In compressed mode, this transmission gap may be repeated. Further, by means of a measurement request or other means, the type of compressed mode may be modified, including the number of slots in a transmission gap, the interval between transmission gaps, and the number of repetitions of transmission gaps (for example, refer to “W-CDMA Mobile Communications System, section 3, Wireless System, 3-3, Wireless Access Interface Standards, m. Compressed Mode” (edited by Keiji Tachikawa, Maruzen Co., Ltd., Mar. 15, 2002, p. 140)).
When uplink compressed mode is set in the above described conventional radio base station apparatus, the slot format of the uplink DPCCH (Dedicated Physical Control Channel) in the physical layer varies greatly from frame to frame as shown in FIG. 6. It is then difficult to make stable determination of uplink reception synchronization. Therefore, there is a need for processing to address this situation.
In particular, the radio base station apparatus use known pilot symbols in determination of uplink reception synchronization. Because the slot format of frames compressed due to compressed mode contains a reduced number of pilot symbols, and further because the number of slots being transmitted is compressed, the number of known pilot symbols per frame is significantly reduced.
Also, the known pilot symbols for compressed frames are made different from those for normal frames. Therefore in compressed mode, determination of uplink reception synchronization is difficult and stable determination of uplink reception synchronization tends to fail. When the radio base station apparatus is in an in-sync state, it performs SIR (Signal to Interference power Ratio) closed-loop power control for providing optimal power control. That is, determination of uplink reception synchronization concerns power control and therefore will significantly affect interferences with other users, the communication quality, and so on.